There is evidence that at frequencies 100 MHz the hotspot spectra may be turning over much more steeply than is allowed by the LEC model. First, the lack of scintillation seen from Cygnus A at \(\sim 80\)  MHz was thought to be due to a rapid turnover in the radio spectra of the compact hotspots [1]}. Second, [2]} estimated the emission from hotspots A and D at 74 MHz by using 10 arcsec-resolution imaging with the VLA. This was done by using a spectral model for the lobe and counter-lobe emission and assuming that the excess flux was due to the hotspots (here we define the lobe as being associated with the brightest radio jet, which is expected to be on the approaching side of the source). They estimated that hotspot D had a flat spectrum consistent with a LEC model, but that the spectrum of hotspot A was decreasing too rapidly towards lower frequencies to be explained by a LEC. An analysis of the lobe spectra by [3]} between 74 and 325 MHz at 25 arcsec resolution (where the beam dilution of the hotspots would be high) found that the lobe and counter-lobes have an asymmetric spectral index, with the counter-lobe (containing hotspot D) having a flatter spectral index than the lobe (containing hotspot A). As we will show, in contrast to the results of [2]} and [3]}, we find the spectrum of hotspot D in the 100 to 200 MHz range to have the more extreme curvature of the two hotspots, with a flatter slope being seen in hotspot A.
